smart_pkt: fix overflow resulting in OOB read/write of one byte

[pks-t]

When parsing OK packets, we copy any information after the initial "ok "
prefix into the resulting packet. As newlines act as packet boundaries,
we also strip the trailing newline if there is any. We do not check
whether there is any data left after the initial "ok " prefix though,
which leads to a pointer overflow in that case as len == 0:

if (line[len - 1] == '\n')
	--len;

This out-of-bounds read is a rather useless gadget, as we can only
deduce whether at some offset there is a newline character. In case
there accidentally is one, we overflow len to SIZE_MAX and then
write a NUL byte into an array indexed by it:

pkt->ref[len] = '\0';

Again, this doesn't seem like something that's possible to be exploited
in any meaningful way, but it may surely lead to inconsistencies or DoS.

Fix the issue by checking whether there is any trailing data after the
packet prefix.

[pks-t]

Note that we already do have a test that exhibits this code path, but none of our CI infrastructure was able to catch the pointer overflow.

[ethomson]

Nice catch, thanks @pks-t